A steam generator is for example known from WO2007/133071 which discloses a single pass evaporator unit which is arranged in a substantially horizontal gas conduit. The evaporator unit has at least one heat transfer section which comprises vertically extending heat transfer tubes. The heat transfer tubes are arranged in a matrix having arrays of heat transfer tubes in a direction transversal to the flow direction of the heating gas and arrays of heat transfer tubes downstream the gas flow. The heat transfer section is in fluid communication with an inlet conduit for supplying a liquid flow medium, generally water, to the heat transfer tubes and an outlet conduit for transferring the flow medium as a two phase mixture of liquid and vapour to a separator. The heat transfer section is bottom fed, which means that the inlet conduit is arranged at a lower region of the heat transfer section. The inlet conduit allows an once through operation of the evaporator section which is necessary to enable operation under supercritical circumstances. The outlet conduit is arranged at an upper region.
The heat transfer tubes are positioned downstream the heating gas flow in the gas conduit. The heating gas passes the subsequent positioned heat transfer tubes which brings a cooling down of the heating gas and a heating up of the heat transfer tubes. A front positioned heat transfer tube is more heated than a back positioned heat transfer tube. The temperature difference between heating gas and the flow medium upstream the gas flow is bigger than the temperature difference between the heating gas and the flow medium in a more downstream positioned heat transfer tube. This normally results in a bigger contribution of the front positioned heat transfer tubes to the heat transfer and the generation of steam. A problem relating to this phenomenon is that a more front positioned heat transfer tube may get damaged by overheating while more back positioned heat transfer tubes do not generate sufficient steam. It is desired to generate steam by the evaporator, wherein all the heat transfer tubes have an approximately equal contribution to the steam generation. It is desired to keep the reduction of the temperature difference within acceptable limits. It is desired that all heat transfer tubes produce an optimum amount of steam.
An additional problem relating especially to the most upstream positioned heat transfer tube, is that the supply of liquid may become too low. A complete evaporation in one or more of the heat transfer tubes of the evaporator unit may impair stability of operation. The heat transfer tube may dry out and become overheated which could lead to damages.
One possible solution to get an optimum contribution of all heat transfer tubes to the steam generation relates to an adjustment of the heat transfer surface of each heat transfer tube. The heating surface of front positioned heat transfer tubes may be enlarged to increase the heat transfer of those tubes. Herewith, an effective contribution of the more back positioned tubes to the steam generation may be achieved.
Such a solution is for example presented in U.S. Pat. No. 6,189,491 which discloses a once-through steam generator in a horizontal type of construction for a through-flow of the heating gas in an approximately horizontal direction. The described once-through steam generator comprises a number of approximately vertically disposed heat transfer tubes, which are commonly connected in parallel for a through flow of a flow medium. The heat transfer tubes are arranged in parallel side by side in the horizontal gas conduit. During use the heat transfer tubes, which are arranged upstream of the heating gas flow will be more heated, than the heat transfer tubes which are arranged downstream. In contrast to a desired equally contribution to the steam production, the most upstream arranged heat transfer tube in the heating gas flow would normally produce the most vapour and would therefore have the largest flow rate of flow medium.
U.S. Pat. No. 6,189,491 provides a possible solution for this problem by optimizing the configuration of the heat transfer tubes in a heat transfer section. The configuration of the heat transfer tubes in the heat transfer section is adapted to compensate for variations in heating in downstream direction of the heat transfer section. Each front positioned heat transfer tube is configured for a higher flow rate of the flow medium than each heat transfer tube disposed downstream of it in the heating-gas direction. A heat transfer tube may have for example a larger inside diameter than a heat transfer tube disposed downstream of it in the heating gas direction. Heat transfer tubes in a region of a relatively high heating gas temperature have a comparatively high flow rate of flow medium. However, this proposed solution results in a more complex and large construction of the heat transfer section. A distribution or a collection element mounted at an end of the heat transfer tubes may for example have a complex configuration to be able to connect it to the varying inside diameters of the heat transfer tubes.
In another proposed embodiment in U.S. Pat. No. 6,189,491 a choke device is connected upstream of a number of heat transfer tubes. The flow rate of the flow medium through the heat transfer tubes can be controlled by the choke device. However, also this solution has not appeared to be satisfying. The configuration of the heat transfer section including a choke device is more complex and susceptible to failures.
GB443,765 discloses a high pressure steam generator. The steam generator includes a tube system which comprises a plurality of temperature stages through which the working medium flows in series. A primary separator is provided between each pair of adjacent stages to separate liquid from steam. The liquid delivered to each primary separator flows therefrom to the next adjacent temperature stage. The delivered steam flows via pipes therefrom to a secondary or main separator common to all stages. With a view to ensure smooth operation of the steam generator in spite of load variations, the ratio of the rate of steam delivery from the primary separators to the rate at which medium is fed to the generator should be maintained substantially constant at any given load. Means for throttling the flow of steam from each of the separators are provided whereby at all loads approximately one fifth of the total quantity of working medium fed to the generator per unit of time at any given load is delivered in the form of steam from each of the separators to the pipes. Thus, the ratio of the quantity of steam delivered from any one separator per unit of time to the quantity of liquid supplied to the generator during that time must be substantially constant at any given load. Means are provided whereby the effective cross-section available for the flow of steam through each throttling device is automatically controlled by a “condition” of the working medium within the system of the generator.
A problem to this configuration of the steam generator is that it includes a plurality of throttling means which makes the configuration more complex and susceptible to failures. The presence of the throttling means increases the flow resistance of the tube system. Another problem is that the common main separator has a complex configuration including a plurality of inlet ports for connecting the pipes originating from the primary separators.
Another problem of the disclosed steam generator is that a two phase mixture of water and steam is fed downwardly through the temperature stages. Generated steam tend to rise in the temperature stage which disturbs the evaporating process. The disclosed steam generator entails stability problems.
EP0.794.320 and EP0.309.792 disclose an exhaust boiler including a high pressure steam generator and a low pressure steam generator. The high and low pressure steam generators comprise each an evaporator unit which operate in separate circuits to generate steam for respectively a low pressure steam turbine and a high pressure steam turbine.
Despite of taken measures, it has appeared in practice that it is still a problem to known steam generators that the evaporator unit appears to be susceptible for overheating. There is still a risk that the front positioned heat transfer tubes deliver a lot of steam and that more back positioned heat transfer tubes in comparison hardly delivers steam. A major problem is still that, the upper ends of the front positioned heat transfer tubes are susceptible for wear and damage as a result of overheating.
A further problem arises when all heat transfer tubes generate superheated vapour having a steam quality equal or larger than one. This may cause temperature stratification over the height of the gas conduit. Heating gas temperature stratification is undesirable because it impairs thermal performance of both the steam generating system itself and possible gas side downstream heat transfer surfaces.